Thermal ink jet print cartridges operate by rapidly heating a small volume of ink to generate a bubble caused by rapid vaporization of an ink vehicle for driving ink through one or more of a plurality of orifices so as to deposit one or more drops of ink on a recording medium, such as a sheet of paper. Typically, the orifices are arranged in one or more linear arrays in a nozzle member. The properly sequenced ejection of ink from each orifice causes characters or other images to be printed upon the paper as the printhead is moved relative to the paper. The paper is typically shifted each time fast and quiet, as only the ink droplet is in contact with the paper. Such printers produce high quality printing and can be made both compact and economical.
A typical ink jet cartridge assembly includes a cartridge body which is attached to a printhead assembly (sometimes referred to hereinafter as "printhead"). Ink which is disposed within the cartridge body flows to the printhead and is expelled in a known manner. More particularly, the cartridge body includes a die cavity in which the printhead is disposed. The printhead is in the form of a nozzle plate attached to a semiconductor chip. A plurality of heater elements are carried by the semiconductor chip, with each heater element being disposed adjacent to a respective nozzle in the nozzle plate. An electrical circuit, which may be in the form of a TAB (Tape Automated Bonding) circuit, electrically interconnects the heater elements with appropriate circuitry in the ink jet printer such that the cartridge elements may be selectively energized as the carriage of the printer travels across the print medium.
The printhead is typically disposed within the die cavity of the cartridge body on a substrate. The silicon chip and nozzle layer are attached to the substrate using a known die attach adhesive. The TAB circuit typically surrounds the printhead and is fastened to the circuit platform of the cartridge using a pressure sensitive adhesive. The TAB circuit includes a plurality of copper leads which extend therefrom and connect with the heater elements on the printhead.
The flexible circuit is typically comprised of a polyimide layer on which copper conductive traces are formed. Preferably, a thin layer of gold is formed on top of the copper conductive traces and provides an amount of protection from inks which can cause corrosion of the circuits. However, for circuits used at higher voltages and higher operating speeds, an epoxy coating or the like is often screened on top of these gold/copper traces to give further protection from inks. The circuit is typically adhered to the printhead and cartridge body by means of a pressure sensitive adhesive, for example an adhesive having an acrylic base.
During routine maintenance of the printhead in the printing process, the nozzle holes are wiped by a wiping mechanism. As a result, ink can travel to the underside of the circuit owing to capillary action of the acrylic-based pressure sensitive adhesive. The pressure sensitive adhesive allows the ink to wick under and be trapped between the circuit and the substrate, causing corrosion or electrical shorting of the flexible circuit in areas with no protective coating. Accordingly, there is a need for improved adhesives which exhibit a reduced ink wicking tendency.
Furthermore, in many applications, precise alignment between the circuit and the print chip are necessary. In these cases, the circuit is in very close proximity or even in soft contact with the substrate during the alignment. The typical pressure sensitive adhesive does not allow the circuit to slip for fine alignment adjustments once it is in contact with the adhesive. One alternative to increase manufacturing efficiency is to provide a release liner on the pressure sensitive adhesive until after alignment and electrical interconnection of the flexible circuit and semiconductor chip. This however is not a reliable process, as removing the release layer potentially can stress the interconnect bonds between the flexible circuit and print chip where the release layer must be removed very close to any TAB bonds.
Another alternative is to insert the adhesive between the circuit and substrate after electrically interconnecting the circuit and the printchip. However, the thin (about 0.001 inch) pressure sensitive adhesive bonding films are often too soft and compliant to be inserted between two surfaces in soft contact.
It would be advantageous to provide a film adhesive which provides good bonding, for example adhering to both a flexible circuit having an epoxy coating and a passivated surface of a substrate, while resisting ink wicking and allowing precise alignment of parts to be bonded. Therefore, there remains a need for an adhesive to attach a flexible circuit to a substrate, for example in an ink jet printhead assembly, which overcomes disadvantages of the prior art.